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Comment by stevep98 | Hacker News Reader

I don't get why everyone keeps talking about the planets aligning. All the planets are pretty much in the same plane. So if you are on one of the planets, all of the others will always be in a line.

edit: to those downvoting me, can you explain why?

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The special occasion is that their phase angles are all of a similar value so they are aligned next to each other. The phase angle describes the position in their respective orbit. If you imagine to look "down" upon the solar system, it would be the angle towards 12 o'clock.

It is rare because the further the planets are from the sun, the longer their orbit periods are. While Earth completes the circle in just a bit above 365 days, Neptune for example takes nearly 165 years to do a round trip. So it would take some time for the slower planets to meet again in the same region in the sky.

No, by being in a plane, the seven planets form a polygon.

If you are in a planet, there is a line to any other planet. Two planets form a line. Three planets form a triangle. Four planets form some irregular polygon of four sides. And so on.

In this case, the area of the polygon formed by the seven planets is minimal, for a period of several years, and they are effectively, almost in a line.

This is not a common occurrence.

I believe the person you're responding to was observing that the planets will appear to be in a line to an observer on the surface of one of those planets.

Polygon? So on? Come on, you know you want to say "heptagon".

I think you're missing the part that from the surface of the Earth, we can never see that whole plane at once. So the special part here is that all the planets are simultaneously in one half of a partition of the plane by a line going through Earth.

At anyone time due to the different radius of their orbits, some planets might be positioned so they would be visible during the day (making them not actually visible). These “alignmnets” mean that they are visible at the same time in the night sky. Them all being on the same plane just means they are located in different spots on the same line in the sky

You are of course right. But for an observer on Earth, planets can be below the horizon. The difference this time is that their orbits are aligning in the other dimension as well, meaning they will be all above horizon when your observation point spins into place.

I didn’t, but I assume they downvoted you as a quick way of saying no to your question, which you chose to phrase in the form of a correct statement.

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